This invention relates to variable optical attenuation (VOA) in a free-space optical switch utilizing micro optical positioning or beam-steering devices.
Communication signals are now commonly transmitted across networks using optical fiber bundles that support substantial bandwidth. Fiber optic networks are composed of a large number of fiber optic lines that can carry many optical signals (e.g., Dense Wavelength Division Multiplexing (DWDM) signals) between nodes in the network. In many instances, optical signals arriving at a node are switched to selected output fibers after some conditioning. The connections between input and output fibers can be made by a variety of cross-connect switches, where any given optical signal on an input line can be switched to any output line at that node.
A typical, all-optical, free-space, cross-connect switch includes an array of optical emitters and an array of optical receivers. Each emitter (or source port) launches a collimated optical beam, which is then coupled into a selected receiver (or target port). collimated optical beam, which is then coupled into a selected receiver (or target port). The emitters can be selectively connected to the receivers by varying the direction of the emitted collimated beam so as to impinge on a selected receiver. In one approach, the emitters launch collimated beams in a fixed direction and use an array of moving micro-mirrors to redirect the beams towards the selected receivers. In an alternative approach, the emitters directly modify the direction of the collimated beam by, for example, tilting a lens or moving a fiber tip behind a lens. With either approach, the receiving port uses a similar mechanism to that of the source port to couple the collimated beam back into the optical fiber. Both types of optical switches can employ Micro-Electro-Mechanical Systems (MEMS) technology, with actuation provided by mechanical, electromagnetic, piezoelectric, photoactive ceramic or polymer, thermal, chemically-active polymer, electrostrictive, shape-memory alloy or ceramic, hydraulic and/or magnetostrictive actuators, or other types of actuators known in the art.
To achieve minimum loss of optical power across the switch, the source and target ports must be aligned accurately to ensure optimal coupling of the collimated beam into the target fiber. Even slight misalignments in either the source or the target will result in a significant loss of power through the connection and, potentially, degradation of the information encoded in the communication signal. Thus, a control system is typically used to correct misalignments actively in the system. Alternatively, it may be desired to attenuate the power coupled into the output fiber. Deliberate attenuation may be desired, for example, in order to equalize the power on multiple channels. Such equalization is often required for proper operation of Erbium Doped Fiber Amplifiers (EDFA). It may also be desired to attenuate power during various equipment tests or to accommodate devices which might be damaged by high power levels.